It is desirable to provide the features and advantages associated with stored program controlled switching to small communities. However, many small communities cannot economically justify the installation of independent, stored program-controlled systems. This situation has led to the development of systems in which a remote switching network can be controlled over a communication link by the central control of a host electronic switching system. One such system, the Western Electric No. 10A Remote Switching System (RSS) described in the Bell System Technical Journal, Vol. 61, No. 4, April 1982, can serve up to 2048 lines and has a stand-alone capability where basic service is maintained for intra-RSS calls in the event of a failure of the host system or the communication link between the host and remote units.
A present trend in switching system design is toward distributed control systems wherein a number of intelligent processors share system decision-making functions. In such systems, the efficiency and reliability of the interprocessor communication are extremely important. For example, the amount of interprocessor communication must be sufficient to assure reliable system operation but must not overburden either the processors or the interprocessor communication arrangement. In the protected environment of a central office, the switching system interprocessor control links can be predefined and no provision for changing their routing within the system is necessary. However, the risk of failure for the control links to a remote switching network is significantly higher. In extending the trend toward distributed control systems to switching systems having a remote switching capability, the problem of maintaining control communication despite a partial failure of the host-remote interconnection facilities and the problem of allocating system decision-making functions that can be distributed to the remote switching entity without reducing system efficiency and reliability are both extremely important.